Electropolishing process and composition



ELECTROPOLISHING PROCESS AND CUMPOSITION Henry George McLeod, St. Catharines, Ontario, Canada,

and Christian John Wernluntl, Niagara Falls, N. Y., assiguors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application January 25, 1951, Serial No. 207,838

Claims. (Cl. 204140.5)

This invention relates to electrolytes of a nitrogenous base plus an organic acid and is more particularly directed to processes and products for electropolishing which employ a substantially anhydrous electrolyte of a nitrogenous base plus an acyclic organic acid plus a dehydrating agent.

The electrolytes of the invention may be used in any of the various ways in which electrolytes have heretofore been employed. They function normally to conduct an electric current between electrodes with a modification of one or both of the electrodes.

The electrolytes of the invention are especially well adapted for the anodic modification and particularly for the electropolishing of metals. The electrolytes may be employed for electropolishing such metals as stainless steel and other alloy steels and non-ferrous metals such as copper and brass.

The technique of anodic polishing of such metals is well understood with other electrolytes and the same methods are generally applicable to the use of the novel electrolytes of the invention. The electrolytes of the invention, however, are unusual because they may be used for the electropolishing of plain carbon steel articles.

The electrolytes of the invention are composed essentially of a salt of a nitrogenous base with an organic acid together with some free acid. The salts can first be prepared and then the acid can be added, but ordinarily it will be simplest to mix the base and the acid in the desired proportions. The acid adds to the nitrogen of the base. Whether a salt forms and precisely the composition of the salt is comparatively unimportant, and for purposes of description the electrolytes will be described as composed of or containing the base and the acid. An

analysis of the electrolyte would apparently show that I moles for each mole of the organic base, though there should be less than about 4 moles of acid per mole of base. For practical operation it is preferred that from gbout 2 to 3 moles of acid be used for each mole of ase.

Any of a wide variety of organic acids may be used in electrolytes of the invention. If an acid melts above normal room temperature then it can be heated for use as an electrolyte, though often the addition of an amine will lower the fusion point s .ciently to avoid the .need for heating. maleic acid, acrylic acid, formic acid, acetic acid, prop'ionic acid, methoxy acetic acid, and phenoxy acetic acid. The acyclic acids are preferred. Acids may be used which contain 'carbocycl'ic or heterocyclic groups.

There may be used, for example, furoic acid, benzoic acid, ,p'henyl acetic acid. phthalic acid, anisic acid, cinnamic acid, para amino benzoic acid, dimethyl amino benzoic acid, .anthranilic acid, .ortho .toluic acid, and cyclo hexane acetic acid. it .will usually be .most convenient to use acids which melt below 100 degrees C.

The acids may also 1b substituted with various groups such as the halogens; chlorine, fluorine, iodine and brominc. Ellhere may be used, for instance, 'difluoroacetic acid, 'triclfloroauetic acid, and similar halogen substituted Such acids may be used, for instance, as

products. Still other groups may be introduced and there may be used for instance, nitrilo acetic acid.

Any of a wide variety of nitrogenous bases may be used in forming electrolytes according to the invention. There may be used, for instance, aniline, phenylethylamine and other aromatic amines containing the benzene ring, having the general formula where R is an aromatic hydrocarbon radical having a single benzene ring, such as, phenyl, tolulyl, Xylyl, or the like and X is hydrogen or an alkyl radical having from 1 tc:1 5 carbon atoms. Monomethyl aniline may also be use Other cyclic nitrogenous bases may be used such as cyclohexyl amine, pyridine, pyrrolidine and morpholine. These latter nitrogenous bases include the nitrogen in a heterocyclic ring. It will usually be most convenient to use bases which melt below degrees C.

The preferred bases for the preparation of electrolytes of the invention are the alkyl amines, and preferably those in which the alkyl group contains from 1 to 5 carbon atoms. The amines may be primary, secondary, or tertiary, tho generally the tertiary amines arepreferred. Such amines include monomethyl amine, dimethyl amine, trimethyl amine, monoethyl amine, diethyl amine, tri-ethyl amine, isobutyl amine, isopropyl amine, triisoamyl amine and the like.

While the discussion has been directed to the use of specific acids and specific nitrogenous bases, it will be understood that mixtures of acids and mixtures of bases may be used.

Electrolytes, according to the invention, are substantially anhydrous. Traces of water may be present, but the water content should not exceed 0.5% by weight. It is even better to maintain the water content still lower and preferably below 0.2% by weight. The electrolytes may be made with anhydrous materials so that they will be free from water.

During operation there is a tendency to pick up water and the water should be removed. This may be done by the addition of a dehydrating agent, such as silica gel, which will absorb the water preferentially.

If the electrolyte boils above the boiling point of water, the water can be driven off by heating the bath. It will also be possible with some electrolytes to remove the water as an azeotrope with one or more of the bath constituents, or even to add a liquid which readily forms an azeotrope. Alcohol or benzene, for instance, can be used for this purpose.

It is easier and more practical, for most purposes, to keep the electrolyte anhydrous by the addition of a dehydrating agent such as acetic anhydride. When the acetic anhydride has reacted with any water present the product is acetic acid and the electrolyte composition is not substantially disturbed. Maleic anhydride and propionic anhydride sirnil arly can be used.

The amount of dehydrating agent or of an anhydride to use is, of course, simply that necessary to keep the bath substantially anhydrous. If acetic anhydride is used, up to about 10% by weight of the total electrolyte can be added. It is preferable, however, to make smaller and more frequent additions.

A preferred composition of the invention is composed essentially of trimethylamine, acetic acid and acetic anhydride. Trimethylamine is added at the rate of about 1 mole for each 2 to 3 moles of acid. The acetic anhydride is preferably present at a concentration from about 0.05 to 1, or more narrowly 0.25 mole.

It is most specifically desirable to use about 0.1 mole of acetic anhydride.

In describing electrolytes of the invention, itjhas been stated that they are composed essentially of a nitrogenous base plus an acid. .The acetic anhydride or other dehydrating agent is, of course, added to assure the anhydrous "condition. New it-will be understood that while the electrolyte is thus constituted, there may be 3 various additives which serve as addition agents to modify the efiects of the bath. This is in accordance with well understood practices.

electrolyte was utilized for the treatment of plain carbon steel articles under the conditions noted. The results obtained are shown in the following table.

Conditions and Kind of Surface Produced Example Electrolyte Moles Operating Condition 1 i Volts 15-40, temp. 18-60 degrees 0., A/SF 00-200.

1.0 4 Volts -40 temp. 18-00 de- 2 Acetic acid 2. 5

{Acetic anhydride. 0.1-0. 35 grees A/SF iTmthylamme'" 1 }Volt 30-45 t 22 57 d a Acetic acid a S I [Acetic auhydride 0. 1 grees A/SF 48 4 Diethylamiue 1.0 Volts 40. temp. 15-80 degrees Acetic aeid. 2.5 0., A/SF 80-400.

5 {Diethyhmine 1.0 }V01ts 25-39, temp. 50-90 de- Acetic acid.......- 2.0 grees 0., A/SF 115-360.

Diethylamine 1.0 Proms 2H5 t $100 d 6 Acetic acid 2.5

{Acetic anhydride 1. 5 grees A/SF 40450 7 {Monomethylammc. 1.0 }Volts 22-41, temp. 22-86 Acetic acid. 2.1 grees 0., A/SF 48-576.

Monomethylaminenu 1.0 li 2040 mm 304,) 8 Acetic acid 2.1

{Acetic anhydride 0.18 Fees A/SF 80492- Tri-isoamylamine 1.0 volts 2744 mm p. -9-98 9 Acetic acid grees 0., A/SF 1.4-119.

Trmhylamine 2 Volts -40 tem -120 Propionic acid 5 f 1 piAcctic anhydride 0.3 grees A/SF {Triethylamme 1.3 }Volts 20-40, temp. 25-80 gormfic iacid grees 0., A/SF 100-400.

Y mine Volts 20-30 temp -40 Formic acid 4. 3 {Acetic anhydridc 0.3 grees A/SF {lgmomcthlylanlhnenu }Volts 2-40i k/tseip pz i 13620-94 ce ic aci grees Aniline 1.0 }Volt.s 24-41, temp. 30-90 Acetic acid goes 0., 11/515 110-345.

w Volts 26-35 temp 25-88 Acetic acid 2.0 Acetic anhydride 35 24311 t e lip gfgi' s 2. 5 grees 0., A/SF 144-237. }Volts 30-45, temp. 25-49 i il l grees 0., A/SF 72-150.

exy Volts 20-44 temp -90 de- 18 Acetic acid 2.0

{fi fi nltlhydride grees 0., A/Sl" 14-82.

F Volts 28-44 temp. 33-77 de- 19 Acetic acid... 2.0

{Acetic anhydride. 0.25 Ewes A/SF 44 20 Morpholine. 1.0 Volts 23-29, temp. 60-90 Acetic ac' 2. 0 grees 0., A/SF 144-192. 21 {Pyrrolidme 1.0 }Volts 20-38, temp. 40-88 .gceticl r2101 2.8 grees 0., 11/81 48-163.

yrro 1 ine 22 Acetic anhydride 0.25 Volts 2%27MteFEdjG53-7Z de- Acetic acid 5 grees Produced a liquid bright finish. Satisfactory operating conditions were found to be in the 25 to 32 v. and 25 to degrees 0. ranges.

The best electrobrightening was secured in the 25 to 30 volt range at around 30 degrees 0.

The best polishing was obtained in the 25 to 40 degrees 0. range. Ex-

cellent results were obtained using direct current at 40 to 45 v. and using alternatin at v. 60 cycle. With A. 0. both electrodes were polished in a minute or two.

Shiny rough etch at low temperature. Rough grayish white etch at high temperatures. Excellent brightening at a narrow temperature range of 12 to 19 de rees 0.

{This electrolyte yielded pleasing bright, smooth elcctropolishes over a wide temp. range. Good deburring also noted. Copper wire also was eleetropolished at 25 volts and 90 to 95 degrees 0.

Electropolishing at low temperature (6 degrees to 10 degrees 0.) and hiah voltage (43 to 44 volts). etches were produced.

At higher temperatures, dull grcy' de- Excellent electropolishing in the 22-30 volt range at 50 to 76 degrees 0.

Good electropolishing obtained at 22 volts and decrees =i:5 (and at de- 40 volts and 36-39 degrees 0.). This electrolyte did not brighten stainless steel. A copper wire was electrobrightened at 18 volts and 80 degrees 0.

de- One anode had an excellent bright finish. The anodes coated with ferric salts in most of the runs.

de- Upper half of anode was bright and lower section was dull at 30 and 40 v. at 50 to 75 degrees 0.

de- Very bright "orange peel finish at 80 degrees 0. Good bright etch at 25 degrees 0. and 21 volts.

de- Excellent electropolishing obtained. Anode current efiiciencieswerc at around 67%.

de- Becst electrobrightening was secured at around 27 volts and 86 degrees de- A very smooth, bright surface was secured. Better at. 35 degrees 0.

than at 60 and degrees 0.

de- The bottom and edges of anode were bright etched.

de- Anodes had both bright and dull areas.

de- At 45 volts, edges were bright and centers were dull; at 30 volts, dull intercrystalline etehes were secured.

de- Excellent brightening; no pitting.

de- This system clectrobrightened at 37 to 40 degrees 0. but produced a dull etch at 80 to 87 degrees 0.

Produced bright finish but salts precipitate on anode causing some littlng. A copper spiral was brightened at 30 volts and 80 degrees The use of the electrolytes of the invention need not be described in detail since the technique of using electrolytes is already well understood. In electropolishing with direct current, an article to be polished serves as an anode in the electrolyte, while another metal, such as copper, bronze, brass, stainless steel, silver, aluminum, Monel metal and other nickel alloys. and even ordinary sheet steel is used as a cathode. When alternating current is used, both electrodes can be polished at the same time. Alternating current gives very good results and for many purposes is preferred. With either alternating or direct current the article polished is an anode for at least part of the time that current is flowing.

The electrolyte can be operated at elevated temperatures below boiling and decomposition points. It is particularly to be observed that when a lower amine, such as monomethylamine, is used the bath tends to be volatile. This is especially true in combination with lower acids, such as formic. Even with acetic acid and trimethylamine there is considerable vapor pressure and an annoying odor. In using the electrolytes accordingly, it may be found desirable to use a closed system, so that vapors will not escape. On the other hand, for small installations it will be sufiicient to provide good ventilation facilities like those used in plating chromium from chromic acid solutions.

The invention is further illustrated by the following examples.

A series of electrolytes were prepared having the compositions indicated in the following tabulation. Each As will be noted from the above examples, there is an optimum range of voltage and of temperature for each different composition of electrolyte wherein the best electropolishing is obtained. As is generally well understood in the art, these ranges vary with the different electrolytes and are easily determined in each case in order to obtain the best results under the specific working conditions.

We claim:

1. An electropolishing process which comprises passing current thru a steel article in an electrolyte containing not more than 0.5 per cent by weight of water and essentially composed of a mixture of about 1 /2 to 4 molar proportions of acetic acid with one molar proportion of an alkyl amine having 1 to 5 carbon atoms per alkyl group.

2. An electropolishing process which comprises passing an electric current thru a steel article in an electrolyte containing not more than 0.2 per cent by weight of water and essentially composed of a mixture of about 2 to 3 molar proportions of acetic acid with one molar proportion of trimethylamine and 0.05 to 1.0 molar proportions of acetic anhydride.

3. An electrolyte composed essentially of a mixture of about 1 /2 to 4 molar proportions of acetic acid with 1 molar proportion of an alkyl amine having from 1 to 5 carbon atoms per alkyl group and from 0.05 to 1 molar proportion of acetic anhydride.

4. In a process for electropolishing steel, copper and brass in which a metal article to be polished is made an electrode, the step comprising passing current through a substantially anhydrous electrolyte composed essentially of a nitrogeneous base selected from the group consisting of alkyl amines having from 1 to 5 carbon atoms per alkyl group, cyclohexyl amine, pyridine, pyrrolidine and morpholine, aromatic amines having the formula where R is an aromatic hydrocarbon radical having a single benzene ring and X is hydrogen or an alkyl radical having from 1 to 5 carbon atoms, and a carboxylic acid which melts below 100 C. there being 1 /2 to 4 moles of acid for each mole of base.

5. In the process for anodic electropolishing of steel, copper and brass in which a metal article to be polished is made an anode, the step comprising passing current through the said article as anode in an electrolyte containing not more than 0.2 per cent by weight of water and essentially composed of a mixture of about 2 to 3 molar proportions of acetic acid with one molar proportion of trimethylamine and 0.05 to 1.0 molar proportions of acetic anhydride.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,836,047 Somerville Dec. 15, 1931 2,288,413 Morgan June 30, 1942 2,338,321 Faust Jan. 4, 1944 2,411,410 Beckwith Nov. 19, 1946 2,565,189 Wernlund Aug. 21, 1951 

4. IN A PROCESS FOR ELECTROPOLISHING STELL, COPPER AND BRASS IN WHICH A METAL ARTICLE TO BE POLISHED IS MADE AN ELECTRODE, THE STEP COMPRISING PASSING CURRENT THROUGH A SUBSTANTIALLY ANHYDROUS ELECTROLYTE COMPOSED ESSENTIALLY OF A NITROGENEOUS BASE SELECTED FROM THE GROUP CONSISTING OF ALKYL AMINES HAVING FROM 1 TO 5 CARBON ATOMS PER ALKYL GROUP, CYCLOHEXYL AMINE, PYRIDINE, PYRROLIDINE AND MORPHOLINE, AROMATIC AMINES HAVING THE FORMULA 